The present invention relates to novel precursors for the production of heterometal oxide films by MOCVD, in particular precursors for the growth of strontium tantalum/niobium oxide films.
The ferroelectric metal oxides (strontium bismuth tantalate (SrBi2Ta2O9 or SBT) and strontium bismuth niobate (SrBi2Nb2O9 or SBN) have a net electric dipole in a certain direction which can be reversed by an applied voltage. These ferroelectric materials retain a remnant polarisation (i.e. charge) even after the power has been switched off which gives them a large potential application in computer technology as capacitor layers in non-volatile ferroelectric random access memories (NVFERAM""s). NVFERAM""s can also be switched extremely rapidly (in hundredths of a nanosecond) and are particularly suitable for military and space applications as they are radiation hard.
Thin films of the layered perovskite strontium bismuth tantalate SrBi2Ta2O9 (SBT), comprising ferro-electric pseudo-perovskite lattices sandwiched between bismuth oxide layers, have a large potential application as capacitor layers in non-volatile ferroelectric computer memories. In contrast to capacitors based on other ferroelectric oxides, such as Pb(Zr,Ti)O3, those based on SBT show negligible polarisation fatigue, are fully compatible with conventional Pt-electrode technology, and maintain good electrical properties, even when very thin.
SBT thin films have been deposited by a variety of techniques including solgel, (Y Ito, M Ushikobo, S Yokoyama, H Matsunaga, T Atsuki, T Yonezawa and K Ogi, Int Ferroelectrics, 1997, 14, 23) metalorganic decomposition (T Mihara, H Yoshirnozi, H. Watanabe and C A Pas de Arnujo, Jpn J Appl Phys, 1995, 34, 5233; T Atsuki, N Soyama, T Yonezawaq and K Ogi, Jpn. J. Appl Phys, 1995, 34, 5096), pulsed laser ablation (P X Yang, N S Zhou, L R Zheng, H X Ln and C L Lin, J Phys. D-Appl Phys, 1997, 30, 527) and metalorganic chemical vapour deposition (MOCVD) (T Ami, K Hironaka, C Isobu, N Nagel, M Sugiyama, Y Ikeda, K Watanabe, A Machida, K Miura, and M Tanaka, Mat Res Soc Symp Proc, 1996, 415, 195; T Li, Y Zhu, S Desu C H Peng and M Nagata, Appl. Phys Lett, 1996, 68, 616). MOCVD has a number of advantages over other deposition techniques as it offers the potential for large area growth, good film uniformity and composition control, and excellent conformal step coverage at device dimension  less than 2 xcexcm. The MOCVD technique is also fully compatible with existing silicon CVD processes.
However for the full potential of MOCVD to be realised, it is essential that precursors with the required physical properties and decomposition characteristics are available. It is important that there is an adequate temperature window between precursor vaporisation and decomposition on the substrate, the precursors need to be compatible and not pre-react, they should decompose to form a pure film of the desired metal oxide at similar substrate temperatures. Ideally, the precursors should also be of low toxicity and be relatively stable under ambient conditions.
The MOCVD of SBT and SBN has thus far been severely restricted by a lack of suitable metalorganic precursors. Conventional precursors include Sr(thd)2(where thd=2,2,6,6-tetramethyl-3,5-heptanedionate), Bi(C6H5)3 and Ta(OPr1)4(thd) which are generally not compatible, having widely differing physical properties and/or decomposition characteristics (M de Keijer and G J M Dormans, MRS Bulletin, 1996, 21, 37). In an effort to alleviate this problem the Sr/Ta heterometal alkoxide [Sr{Ta(OPr)6}2] has been investigated as a precursor to SBT, in combination with Bi(OBu1)3. A potential advantage of this approach is that the strontium and tantalum ratio in the precursor matches the required ratio in the deposited SBT film, however, there exists the possibility that the strontium and tantalum alkoxide species will partition during precursor evaporation and transport. Another disadvantage is that [Sr{Ta(OR)5}2] precursors are relatively unsaturated making them susceptible to attack by moisture and reducing their shelf life in solution-based liquid injection MOCVD.
EP-A-0 807 965 (Matsushita Electronics Corporation) Nov. 19, 1997 describes a method of forming a Bi-layered ferroelectric thin film on a substrate, using a mixed composition of a Bi-containing organic compound and a metal polyalkoxide compound.
It is an aim of the present invention to provide new metalorganic precursors for the MOCVD of SBT and SHN which may overcome the above-mentioned drawbacks.
According to one aspect of the present invention there is provided a metalorganic precursor of the formula:
Sr[M(OR1)6-xLx]z
wherein x is from 1 to 6;
M is Ta or Nb; R1 is a straight or branched chain alkyl group; and
L is an alkoxide group of the formula: 
wherein n=0 or 1; X is N or O; R2 and R3 are the same or different and are straight or branched chain alkyl groups, and R4 is a straight or branched alkyl chain, optionally substituted with an amino, alkylamino or alkoxy group.
According to a second aspect, the present invention provides a method of depositing thin films of or containing strontium metal oxides using metalorganic precursor in a MOCVD technique, wherein the strontium metal oxide precursor has the formula:
Sr[M(OR1)6-xLx]z
wherein x is from 1 to 6; M is Ta or Nb;
R1 is a straight or branched chain alkyl group; and L is an alkoxide group of the formula: 
wherein n=0 or 1; X is N or O; R2 and R3 are the same or different and are straight or branched chain alkyl groups and R4 is a straight or branched alkyl chain, optionally substituted with an amino, alkylamino or alkoxy group.
Preferably, x is 1 or 2.
The deposition, technique may comprise conventional MOCVD or, more preferably, liquid injection MOCVD. The solvent for deposition of the films by liquid injection MOCVD is preferably tetrahydrofuran.
The alkoxy group OR1 is preferably an ethoxy group but compounds of the invention where OR1 is, for example, an iso-propoxy or tertiarybutoxy group may also be useful. Preferred precursors of the invention have the formula:
Sr[M(OR1)5L]2 or Sr[M(OR1)4L2]2
wherein M, R1 and L are as defined above.
Preferably L is a dimethyl aminoalkoxide group, particularly dimethyl aminoethoxide (OCH2CH2NMe2 or DMAE), dimethyl aminopropoxide (OCH(CH3)CH2NMe2 or DMAP) or bis-dimethyl aminopropoxide (OCH(CH2NMe2)CH2 NMe2 or bis-DMAP). Alternatively, L may be an alkoxy alkoxide group, particularly xe2x80x94CH2CH2OMe, xe2x80x94OCH(CH3)CH2OMe or xe2x80x94OCH(OMe)CH2OMe.
The above-mentioned precursors may also be used in combination with a variety of bismuth (Bi) sources to deposit strontium bismuth tantalum and strontium bismuth niobium metal oxides.
Suitable precursors for the source of bismuth include triphenyl bismuth (Bi(C6H5)3, Bi(thd)3, Bi(OCH2CH2NMe2)3 and Bi(OCMe2CH2OMe)3.
The Bi(OCMe2CH2OMe)3 precursor is particularly suitable as a co-precursor, being one of the most stable and volatile Bi alkoxide sources available.
The Bi precursors may be evaporated separately or may be combined with the Sr[M(OR1)6-xLx]2 is a single solution. In the latter case, the bismuth precursor may have the general formula BiL3, wherein L is a dialkyl aminoalkoxide or alkoxy alkoxide group as hereinbefore described in relation, to the strontium metal oxide precursor. Preferably, the dialkyl aminoalkoxide or alkoxy alkoxide group of the bismuth precursor is the same as that of the strontium metal oxide precursor. The single solution may be in an organic solvent such as ether or cyclic ether (eg. THF) or a hydrocarbon, such as hexane or heptane.
The precursors of the present invention may be used in a method for depositing a strontium metal oxide ferroelectric film onto a substrate by MOCVD. A suitable substrate is, for example, Si(100). The ferroelectric films may be used, in particular, for the production of non-volatile ferroelectric random access memories.
The use of MOCVD precursor solutions containing mixtures of metal, alkoxides with nitrogen or oxygen donor functionalised ligands such as OCH2CH2NMe2 or OCH2CH2OMe can be readily extended to other oxide and mixed oxide systems. It has recently been shown that the dielectric constant of bulk Ta2O5 can be significantly increased by the addition of a small percentage of TiO2. This offers the potential for improved performance Ta2O5xe2x80x94based DRAM""s (dynamic random access memory). The precursor solutions described herein are likely to be appropriate for use in the MOCVD of the mixed Ta2O3/TiO2. A suitable precursor combination is Ta(OR)4DMAE and Ti(OR)2(L)2 where R is preferably Et or alternatively may be Pri, Prn, Bur, Bun etc., and L is DMAE, DMAP or bis-DMAP etc.
This invention will be further described, by way of example only, with reference to the accompaying drawings in which:
FIG. 1 illustrates the molecular structure of the novel precursor,
Sr[Ta(OEt)5(bis-DMAP)]2;
FIG. 2 is a plot of growth grates against substrate temperature achieved by MOCVD using two strontium tantalum precursors of the present invention;
FIG. 3 is a plot of growth rates against injection rate achieved by MOCVD using two strontium tantalum precursors of the present invention; and
FIG. 4 is a plot of growth rates against oxygen flow achieved by MOCVD using two strontium tantalum precursors of the present invention.